They feared that chemicals and waste rushing down the Mississippi would result in the largest-ever oxygen-depleted “dead zone” measured in the gulf since monitoring began in 1985.

New results are in: on Monday, the National Oceanographic and Atmospheric Administration announced that a team of scientists mapping the dead zone had just returned from a midsummer research cruise. The zone was mapped at 6,765 square miles — above average, but not as large as the 8,500 to 9,400 square miles predicted earlier. In fact, this year’s dead zone is only the 11th-largest of those recorded in the last 20 years.

But it is hardly time for a collective sigh of relief, according to Nancy Rabalais, chief scientist of the Louisiana Marine Consortium, who led the research effort. She emphasized that Tropical Storm Don had swept through the gulf as the research team was collecting data last week, stirring up the otherwise stratified waters and at least temporarily supplying oxygen to formerly depleted areas.
Dr. Rabalais said there was therefore some uncertainty about how accurately the mapping data represented the true health of the gulf. And it is certain, she said, that the levels of nitrogen and phosphorus that cause deadly oxygen depletion in the water continue to rise year by year.

“We know that pollution is increasing and the gulf system is slowly, over time, losing its resilience,” Dr. Rabalais said. “It doesn’t take nearly as much to aggravate the system as it did before, and it will take longer to recover.”

“The tricky thing about mapping the dead zone like this is that you’re really just taking a snapshot of what’s going on at the moment,” he said. “Tropical Storm Don came in right where they predicted there would be a high level of hypoxia and temporarily mixed up the water, but it may settle back soon.”

While the dead zone appears to be much smaller than feared, Mr. Rota also pointed out that it is still much larger than the target set by The Mississippi River/Gulf of Mexico Nutrient Task Force, an organization that has pledged to reduce the affected area to 1,930 square miles.

“This number is almost certainly an underestimation of area actually affected and even if the number was accurate, it would not be an indication that we are adequately addressing the problem,” Mr. Rota said. “The average for the last five years is higher than the previous five years. Clearly we’re still trending in the wrong direction.”

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